Ecosystem services, the benefits people receive from ecosystems, provide a framework for organizing and reinterpreting biophysical processes, often ones scientists are already studying, in the context of impacts on people. Thus, the nutrient retention provided by an agricultural buffer strip is evaluated by the improvement to drinking water in a downstream community or the increased swimability of a local lake. Hydrologic ecosystem services relate to the impacts on people that stem from ecosystem effects on water flow.

In many ways, hydrologic ecosystem services are a reframing of agricultural best management practices and other techniques conceived by agronomists and soil scientists. The exciting and novel aspect of the ecosystem services framework, especially for hydrologic services, is the focus on specific beneficiaries. Ecosystem services assessments evaluate who is affected by an action, and how much. In addition, ecosystem services facilitate bundling and comparison among services, so that the effects of agricultural management on, for example, both water quality and native bird abundance, can be evaluated. Finally, as a result of the focus on beneficiaries, ecosystem services provide an opportunity for payments for ecosystem services (PES).

We illustrate these principals with a case study from the Big Island of Hawai’i. Here, conversion among ranchland, commercial silviculture, and old-growth forest have diverse impacts on groundwater recharge. We measured changes in water flux expected from these transitions and then modeled the expected impact on the pumping depth to the drinking water aquifer. The direct interception of fog water played a controlling role on the water cycle in this location, and measurable impacts to the local Department of Water Supply are predicted. However, in this humid, groundwater-dominated system, we found that the hydrologic effects were relatively small compared to the potential value of carbon storage, biodiversity, and direct income from the same land use transitions.